A number of processes are possible for making colloidal sol, however, most of these processes result in particle sizes that have multiple peaks creating a wide disparity of particle sizes. Currently, there is a high demand for high-purity colloidal sol containing few impurities for various uses, particularly in the electronics industry. Colloidal sol may be used for a variety of processes including final polishing of wafers, such as silicon wafers for semiconductor devices, because high-purity colloidal sol does not contaminate the silicon wafers. A colloidal sol with small silica particles has a high precision capability of polishing materials that are required to be scratch-free and microstructural. Thus, a highly concentrated colloidal sol with generally consistent single sized peak distribution of small silica particles, wherein the size of the silica particles is precisely controlled is needed. Current methods of making a colloidal sol create as different sized particles, which provide different polishing properties, and most colloidal sol has at least two peak distributions of particle sizes, as illustrated in FIG. 1. More specifically, colloidal sol having known particle sizes is desirable because the polishing characteristics of the colloidal sol may be repeatedly used in polishing high precision silicon wafers, with consistent results.
While a number of processes exist for producing colloidal sol, most of them have three major problems. The first problem, as stated above, is that most colloidal sols result in multiple particle size peak distributions, i.e., the particle sizes when graphed for intensity versus size, as illustrated in FIG. 1, include at least two peaks, such that typically at least two large peaks of particle sizes occur. Also, as illustrated in FIG. 1, most current methods create one peak for larger particles and one peak for smaller particles with one peak being larger than the other peak. While these dual-peak particle size compositions may be repeatedly made, which allows some consistency in polishing, they are not as desirable as single-peak particle size composition for use in polishing. Even if a single peak size distribution is obtained for current silica sols, the center point of the size distribution peak varies from batch to batch with the current silica sols at best being able to obtain a size of ±10 nm, which for 50-70 nm particles (a common size) is up to a 20% variance in sizes, which creates substantially different performance characteristics.
Second, the size of the particles is typically controlled to provide desired polishing characteristics; however, many of the processes for controlling particle size of colloidal sol use ingredients that are considered impurities when used in combination with semiconductor devices. As such, these processes are not desirable for colloidal sols that are to be used with semiconductor devices. Trace metals may also cause undesirable and variable polishing characteristics.
Third, most colloidal sol products have the tendency to grow in particle size with time, which creates unexpected polishing characteristics. The rate of growth may vary inconsistently depending on storage and transportation conditions, and time since manufacture, which causes inconsistencies in polishing characteristics for even colloidal sols of the same composition and even sometimes from the same batch. Therefore, all factors from storage to transportation to storage at the end use facility must be strictly controlled to ensure consistent quality, which is very expensive. In addition, it is difficult to order large quantities and store such quantities use and draw out of storage over a time period due to the tendency of the particles to grow in size with time. Further, many of the attempts to control particle size and reduce growth characteristics after manufacture of the colloidal sol, increase the viscosity of the colloidal sol which reduces the ability of the colloidal sol to effectively polish semiconductors.
Other issues do exist with most silica sols that are currently available. Many silica sols include sodium, which is considered an undesirable impurity for many uses in the electronics industry. Many silica sols also require a narrow range of specific storage and transportation temperatures so that the quality of the sol is not degraded. Current sols experience quality issues when the temperature falls below 5° C., and the silica sol should not be allowed to freeze. These temperature limitations may require heated or cooled transportation, which is difficult or expensive.
In view of the above problems, this invention provides a high-purity, highly concentrated colloidal sol with long-term stability, long-term particle size stability and low viscosity, allowing for long-term storage after production and consistent performance characteristics. The invention also provides the ability to control particle size to be predominately a single peak of particle sizes, thereby allowing products of exceptional polishing properties for the semiconductor industry.